Labour is busy accentuating the negative while National seems hell bent on eliminating the positive with less than a week to go to the Christchurch East by-election.

While Saturday's poll will determine whether Poto Williams replaces Mayor Lianne Dalziel in the traditionally Labour bastion, both parties are jockeying to win the pre-vote positioning.

At stake is the "narrative" - whether the final result is seen as a victory for their side.

It's a classic political ploy - under promise and over deliver so you can claim a victory even from a poor showing.

Facing an effective public referendum on David Cunliffe's leadership, Labour wants to downplay expectations in case a lacklustre showing damages his standing.

So despite Dalziel's solid 5334 majority in 2011, Cunliffe has been talking up National's equally emphatic victory in the party vote in 2011, by 13,252 (46 per cent) to 9100 for Labour (31.65 per cent).

Labour's "key message" is that the seat is National's to lose.

For Cunliffe "any old win would do", he said yesterday.

"I would say 50 per cent would be great."

He is adamant the party vote is the best measure of "underlying party allegiance" available.

Meanwhile, Prime Minister John Key has been pointing to history to argue a victory is a long shot.

National has never won the seat and no government has ever won a by-election in a seat it did not already hold. So by implication even a good showing would be a win. A narrow victory would be a triumph.

"The probability of that is low," he said.

And he has set an extremely low bar for success. "As long as the majority reduces we'll be happy."

The reality is somewhere in between.

National did win a thumping victory on the party vote, but that was in the context of a huge lead over Labour everywhere. And because the Left vote was split by the strong showing of the Greens, Labour struggled to carry the party vote in many seats.

Even in Cunliffe's New Lynn seat National out-polled Labour by 39.22 per cent to 37 per cent in 2011.

In Christchurch East the Greens took 11.68 per cent and NZ First 6.26 per cent.

So even when National was riding high - and with its minor parties allies the Conservatives, UnitedFuture and Act factored in - the Government-Opposition divide was even.

Also, Labour's polling is about 5 percentage points higher now than it was on election night.

So there is no reason why a strong party vote then should translate into a win in the seat.

Dalziel's personal popularity will be a factor. Her majority was 5334 - 15,559 votes (53.54 per cent) to 10,225 (36.50 per cent) - over the luckless Aaron Gilmore. She has endorsed Williams.